This invention relates to dual hydraulic booster assemblies for vehicle hydraulic braking systems of the kind comprising a pair of pedal-operated hydraulic boosters which are adapted to be operated simultaneously for applying brakes on wheels on opposite sides of a vehicle for vehicle retardation and independently for applying a brake on a wheel on one side of the vehicle to facilitate steering.
Dual hydraulic booster assemblies have application to braking systems for vehicles where it is desirable to achieve braking on front and rear wheels for normal service braking but to restrict braking to one rear wheel on one side only of the vehicle for steering purposes and in particular, but not exclusively, agricultural vehicles such as tractors which may tow trailers.
It is an object of the present invention to provide a dual hydraulic booster assembly which can be used for such application.
It is a preferred object of the present invention to provide a dual hydraulic booster assembly in which the fluid pressure obtained when both boosters are operated simultaneously is increased for braking on both sides of the vehicle.
According to one aspect of the present invention there is provided a dual hydraulic booster assembly for a vehicle braking system incorporating a logic valve responsive to booster pressures to control the supply of pressure fluid for braking on one side of a vehicle when one booster is operated on its own in a first mode of operation, and for braking on both sides of a vehicle when both boosters are operated simultaneously in a second mode of operation.
By this invention, the logic valve controls the supply of pressure fluid so that, in the first mode of operation, when one booster is operated, the supply of pressure fluid causes braking of one rear wheel on one side of the vehicle associated with that booster to facilitate steering and, in the second mode of operation, when both boosters are operated simultaneously, the supply of pressure fluid causes braking of the wheels on both sides of the vehicle for normal service braking.
More particularly, the logic valve controls the supply of fluid pressure in the second mode of operation when both boosters are operated to cause braking of the front wheels of the vehicle and a trailer (where provided) with the boosters causing braking of the rear wheels of the vehicle. In this way, all wheels are braked in the second mode of operation. By contrast, the logic valve isolates the supply of fluid pressure from the brakes for the front wheels and trailer (where provided) in the first mode of operation when only one booster is operated so that only the rear wheel associated with that booster is braked.
The dual hydraulic booster assembly of the present invention has the advantage that the logic valve is common to both boosters in an integrated unit that avoids the requirement for auxiliary logic heads and valves separate from the booster assembly. As a result, the number of parts and associated pipework and fittings are reduced to a minimum thereby facilitating installation and simplifying repair/maintenance.
Preferably, each booster comprises a boost piston working in a bore in a body adapted to generate an output force for operating a hydraulic master cylinder coupled to the rear brakes on one side of the vehicle in response to pressure in a boost chamber which is applied to a proportional power pressure chamber under the control of a control valve, in turn responsive to an input force from the pedal.
Advantageously, each booster and associated master cylinder are combined and both booster and master cylinder assemblies are integrated in a common housing with the outputs from the master cylinders arranged to operate the rear brakes on opposite sides of the vehicle. For example, each master cylinder may operate one rear brake such that a braking force may be applied to the rear wheel on that respective side only to facilitate steering.
In one construction the logic valve comprises inner and outer pressure-responsive telescopic pistons working in a bore in the body and responsive to pressure applied to the outer ends by the associated booster to control the supply of pressure fluid in the first and second modes of operation.
Preferably, the pistons are held in a passive condition of the logic valve by a first spring acting between the pistons, and by a second spring acting on the outer piston only with respective pre-loads of the springs being chosen to withstand the collective threshold forces of seals on each piston to ensure full return of each piston to the passive condition upon release of the pressure applied to the outer end.
Advantageously, when both boosters are actuated simultaneously in the second mode of operation, displacement of both pistons relative to each other allows flow through passages leading from the boosters to a pressure chamber defined between the pistons to activate a braking system for the front wheels on both sides of the vehicle and trailer brakes (where provided), with the passages being isolated from the pressure chamber in both the passive condition and when either booster is operated on its own in the first mode of operation.
More particularly, relative displacement of the pistons provides flow paths through the pressure chamber for the supply of pressure fluid to a common outlet port in the second mode of operation to activate the braking system for the front wheels and trailer brakes (where provided), and to isolate the outlet port in the first mode of operation.
In this way, the front brakes/trailer brakes are only applied when both boosters are actuated and, when either booster is operated on its own, the front brakes/trailer brakes are isolated so as to apply only the respective rear brake coupled to that booster.
Preferably, the logic valve is also provided with oppositely acting one-way return valves which are normally open in the passive condition and are closed to prevent fluid flowing from the pressure chamber back to the boosters when the pressure chamber is pressurised and which open again to return fluid to a reservoir upon release of the boost pressure at the end of the braking cycle.
Advantageously, the pistons are displaced by the full boost pressure from the associated booster which acts to close the one-way valves and, when both boosters are operated together, the pressure chamber is connected to the proportional pressure in the proportional power pressure chambers of both boosters which never exceeds the full boost pressure to ensure the one-way valves remain closed.
At the knee-point of the booster assembly, when the proportional pressure equals the full boost pressure, the logic valve is held in equilibrium and the springs urge the pistons into the passive position, but with full boost pressure applied through the pressure chamber.
The performance of the dual hydraulic booster assembly with logic valve is satisfactory for most applications but there can be situations for normal service braking when the front and rear braking requirement is such that a higher fluid pressure is required than the pressure conventionally available on the vehicle.
Accordingly, in a preferred arrangement, when both boosters are operated simultaneously, the pressure of the pressure fluid is increased by the provision of a pressure amplifier valve to enable higher fluid pressures to be obtained for the braking requirements under normal service braking.
The pressure amplifier valve may be provided separate from the booster assembly so as to receive the pressure fluid from the booster assembly via the logic valve in the second mode of operation. This adds to the overall cost of the braking system, in particular, the installation and maintenance costs for providing a separate pressure amplifier valve.
More preferably, however, the pressure amplifier valve is incorporated into the booster assembly to produce a single integrated unit. In this way, installation and maintenance is simplified compared to the provision of a separate pressure amplifier valve with resulting potential cost savings.
More particularly, the number of separate parts and components is reduced by integrating the amplifier valve into a combined dual hydraulic booster assembly with logic valve.
By this combination of the pressure amplifier valve with the logic valve, the logic valve controls the supply of fluid pressure to provide separate, independent braking on each side when only one booster is operated to facilitate steering, and simultaneous, combined braking on both sides when both boosters are operated together for full vehicle retardation during normal service braking, with the pressure amplifier valve being operable to ensure the required fluid pressure is obtained when both boosters are operated simultaneously.
More particularly, the dual hydraulic booster assembly operates via the logic valve either to connect the pressure amplifier valve to actuate the front brakes of the wheels on both sides of the vehicle and, where provided, the trailer brakes, while conventionally boosting the master cylinder pressure to actuate the rear brakes on both sides of the vehicle for normal service braking when both boosters are operated simultaneously, or to isolate the pressure amplifier valve to actuate the rear brakes on one side of the vehicle only for steering when only one of the boosters is operated.
Preferably, the amplifier valve comprises a piston working in a stepped bore to define chambers of different area for increasing the proportional pressure from the logic valve in a ratio according to the quotient of the areas.
Advantageously, the piston is responsive to the proportional pressure from the logic valve to displace the piston against a return force and pressurise fluid in an outlet chamber connected to the braking system for the front brakes/trailer brakes.
Preferably, the outlet chamber is connected to the reservoir in the retracted position to relieve the pressure in the outlet chamber when the brakes are released.
According to another aspect of the present invention, there is provided a dual hydraulic booster assembly for vehicle hydraulic braking systems comprising a pair of pedal-operated hydraulic boosters adapted to be operated independently for applying a brake on a wheel on one side of the vehicle to facilitate steering in a first mode of operation and simultaneously for applying brakes on wheels on opposite sides of a vehicle for vehicle retardation in a second mode of operation, and a pressure amplifier valve for increasing the fluid pressure obtained in the second mode of operation when both boosters are operated simultaneously.
Preferably, a logic valve responsive to booster pressures is operable to isolate the pressure amplifier valve from the boosters when one booster is operated on its own in the first mode of operation, and to place the pressure amplifier valve in fluid communication with the boosters when both boosters are operated simultaneously in the second mode of operation.
Advantageously, the pressure amplifier valve and logic valve are incorporated with the boosters in an integrated unit in which the logic valve places the pressure amplifier valve in fluid communication with pressure fluid from both boosters in the second mode of operation.
According to yet another aspect of the present invention, there is provided a dual hydraulic booster assembly incorporating a logic valve and a pressure amplifier valve, the logic valve being responsive to booster pressures to isolate the pressure amplifier valve from both boosters when one booster is operated on its own in a first mode of operation, and to place the pressure amplifier valve in fluid communication with both boosters when both boosters are operated simultaneously in a second mode of operation.
Preferably, the boosters provide independent braking for the rear wheels on opposite sides of the vehicle in the first mode of operation, and simultaneous braking for the front wheels on both sides of the vehicle and for the wheels of a trailer where provided in the second mode of operation.
With this arrangement, the fluid pressure from each booster provides braking on one side of the vehicle for steering in the first mode of operation with the pressure amplifier valve isolated, and the fluid pressure from both boosters is increased by the pressure amplifier for braking on both sides of the vehicle in the second mode of operation.
The invention will now be described in more detail, by way of example only with reference to the accompanying drawings.